Method for Coating a Substrate and Metal Alloy Vacuum Deposition Facility

ABSTRACT

The present invention provides a process for coating a substrate. A metal alloy layer including at least two metallic elements is continuously deposited on the substrate by a vacuum deposition facility. The facility includes a vapor jet coater for spraying the substrate with a vapor containing the metallic elements in a constant and predetermined relative content, the vapor being sprayed at a sonic velocity. The process may advantageously be used for depositing Zn—Mg coatings. The invention also provides a vacuum deposition facility for continuously depositing coatings formed from metal alloys, for implementing the process.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 12/532,043 filedDec. 31, 2009, the entire disclosure of which is hereby incorporated byreference herein.

The present invention relates to a process for continuously coating asubstrate and to a vacuum deposition facility for coatings formed frommetal alloys, such as for example zinc-magnesium alloys, said processbeing more particularly intended for coating steel strip, without in anyway being limited thereto.

BACKGROUND

Various processes for depositing metal coatings composed of alloys on asubstrate, such as a steel strip, are known. Among these, mention may bemade of hot-dip coating, electrodeposition and also the various vacuumdeposition processes, such as vacuum evaporation and magnetronsputtering.

Thus, a vacuum evaporation process, described in WO 02/06558 is knownthat consists in coevaporating two elements in a chamber so as to mixthe vapor of the two elements together before coating the substrate.

However, industrial implementation of this process is difficult and isnot conceivable for production that must guarantee a stable coatingcomposition over long substrate lengths.

It is also possible for a layer of each of the constituent elements ofthe alloy to be deposited in succession on the substrate and then tocarry out a diffusion heat treatment resulting in the formation of analloyed layer having the most homogeneous composition possible. Thus, inparticular zinc-magnesium coatings may be produced which mayadvantageously be used instead of coatings of pure zinc or other zincalloys.

This successive deposition of each of the elements may in particular becarried out by vacuum co-evaporation of each element placed in aseparate crucible, as described in EP 730 045, but also by vacuumdeposition of an element on a strip precoated with another element by aconventional hot-dip process.

However, the subsequent diffusion heat treatment may prove to becomplicated and expensive as it involves the use of large quantities ofinerting gas in order to prevent any oxidation of the coating at hightemperature during the heat treatment. Furthermore, to avoid any risk ofoxidation between the magnesium coating and the start of the diffusiontreatment, it is necessary to perform the two operations one immediatelyafter the other, without exposing the strip to the open air.

This heat treatment may also pose problems in the case of certainmaterials that are not compatible with an excessively large temperaturerise. Mention may in particular be made of bake-hardening steel stripwhich contains large amounts of carbon in solid solution, which must notprecipitate before the strip has been formed by the user of thematerial.

Moreover, in this type of process, it is very tricky to obtain a coatingof constant composition over a long substrate length as it is necessaryfor the thicknesses of each layer to be very precisely controlled overthe course of time.

Finally, the diffusion treatment does admittedly allow the alloy toform, but it may also lead to the diffusion of elements from thesubstrate to the coating, thus contaminating the interface with thesubstrate.

SUMMARY

An object of the present invention is therefore to remedy the drawbacksof the processes and facilities of the prior art by providing a vacuumdeposition facility for depositing coatings formed from metal alloys anda process for manufacturing a metal strip covered with a metal alloylayer, which allow simple industrial implementation, in few steps, butwhich also allow a coating of constant composition to be obtained, onvarious types of substrates.

The present invention provides a process for coating a substrate,whereby a metal alloy layer comprising at least two metallic elements iscontinuously deposited on said substrate by means of a vacuum depositionfacility comprising a vapor jet coater for spraying the substrate, at asonic velocity, with a vapor containing said at least two metallicelements in a constant and predetermined relative content, said vaporbeing obtained by evaporating a metal alloy bath containing saidmetallic elements in a predetermined initial content, said initialcontent of the bath being kept constant during the deposition.

The process according to the invention may also comprise variousfeatures, taken by themselves or in combination, as follows:

-   -   the metallic elements are zinc and magnesium;    -   the metal alloy layer contains no iron-zinc intermetallic        phases;    -   the metal alloy layer predominantly consists of a Zn₂Mg phase;    -   a layer of a zinc-based metal alloy having a predetermined        magnesium content of between 4% and 20% by weight is        continuously deposited on the substrate by evaporating a bath of        a zinc-based metal alloy initially having a predetermined        magnesium content of between 30% and 55% by weight of magnesium,        the initial content being kept constant during the deposition;    -   a layer of a zinc-based metal alloy having a predetermined        magnesium content of between 4% and 18% by weight is        continuously deposited on the substrate by evaporating a bath of        a zinc-based metal alloy initially having a predetermined        magnesium content of between 30% and 50% by weight of magnesium,        the initial content being kept constant during the deposition;    -   the metallic elements have evaporation temperatures differing by        no more than 100° C. at the selected evaporation pressure;    -   a metal alloy layer is deposited with a thickness of between 0.1        and 20 μm;    -   the substrate is a metal strip and preferably a steel strip;    -   the metal strip is made of a bake-hardening steel; and    -   the metal alloy layer consists predominantly of a Zn₂Mg phase.

The present invention also provides a vacuum deposition facility forcontinuously depositing coatings formed from metal alloys comprising atleast two metallic elements on a running substrate, comprising a vacuumdeposition chamber and means for running the substrate through thischamber, the facility further comprising:

-   -   a sonic vapor jet coater;    -   means for feeding said coater with vapor comprising said at        least two metallic elements in a predetermined and constant        ratio;    -   means for evaporating a bath of metal alloy comprising said        metallic elements, which will feed said coater; and    -   means for adjusting the composition of the metal alloy bath,        enabling it to be kept constant over the course of time.

The facility according to the invention may also comprise the followingvariants, taken in isolation or in combination:

-   -   the means for adjusting the composition of the metal alloy bath        comprise means for feeding the evaporation means with a molten        metal alloy of controlled composition;    -   the evaporation means consist of an evaporation crucible        provided with heating means and said means for feeding said        evaporation crucible with a molten metal alloy of controlled        composition comprise a recharging furnace which is connected to        metal ingot feed means and is provided with a heating system,        said recharging furnace being connected to the evaporation        crucible that it feeds;    -   the facility further includes means for continuously circulating        the bath, in the form of a recirculation pipe connecting the        evaporation crucible to the recharging furnace;    -   the evaporation crucible is placed in the vacuum chamber and the        recharging furnace is placed outside the vacuum chamber;    -   the recharging furnace and the evaporation crucible are placed        side by side and have a common wall pierced by at least one        opening located beneath the level of the metal alloy bath but        above the bottom of the furnace and of the crucible; and    -   the evaporation crucible is placed in a confined chamber and the        recharging furnace is placed outside the confined chamber.

The present invention further provides an ingot based on zinc containing30 to 55% magnesium by weight, preferably 30 to 50% magnesium by weight,and able to be used for implementing the process according to theinvention or in a facility according to the invention.

The present invention includes depositing a metal alloy of givencomposition on a substrate by a sonic vapor jet coating process.

Owing to the pressure difference created between a closed evaporationcrucible and the deposition chamber, it is possible to generate, througha narrow slot, a metal vapor jet of possibly sonic velocity, see forexample, WO 97/47782, hereby incorporated by reference herein for afuller description of the details of this type of device.

The vapor feeding the JVD (Jet Vapor Deposition) device comes from thedirect vacuum evaporation of a bath of the alloy itself, the compositionof the bath being kept constant over the course of time.

Now, taking the example of a zinc-based alloy containing magnesium, eachof these two elements has a different vapor pressure. The composition ofthe layer deposited will therefore not be the same as that of the ingotused as raw material for the evaporation. Thus, as may be seen in FIG.1, which shows the magnesium content in wt % in the coating plotted onthe y-axis as a function of the magnesium content in wt % in the bathplotted on the x-axis, to obtain a magnesium content of 16% in thecoating it is necessary to have 48% magnesium in the metal bath.

Because of this difference in the vapor pressures of the alloy elements,the composition of the alloy bath used for the evaporation and, in fact,the corresponding vapor flux will vary over the course of time, with inthe case of zinc-magnesium a progressive enrichment with magnesium.

To keep the composition of the evaporation flux constant over the courseof time, it is necessary to provide a device enabling the composition ofthe bath to be kept constant if it is desired to be able to deposit thistype of coating in the context of industrial implementation.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent onreading the following detailed description given solely by way ofexample, with reference to the appended figures in which:

FIG. 1 shows the magnesium content in wt % in a ZnMg coating as afunction of the magnesium content in wt % in the liquid metal bathbefore evaporation;

FIG. 2 shows a first embodiment of a facility according to theinvention;

FIG. 3 shows a second embodiment of a facility according to theinvention; and

FIG. 4 shows the microstructure of a 5 μm coating of ZnMg alloydeposited on a cold-rolled low-carbon steel.

DETAILED DESCRIPTION

The description that follows will refer to a coating of a zinc alloycontaining magnesium, but it is quite obvious that the facilityaccording to the invention is not limited thereto and that it ispossible to deposit many other coatings based on metal alloys.

A first embodiment of a facility according to the invention is shownmore particularly in FIG. 2, which shows a facility 1 comprising avacuum deposition chamber 2. This chamber 2 is preferably kept at apressure of between 10⁻⁸ and 10⁻⁴ bar, for example. It has an entryload-lock and an exit load-lock between which a substrate S, such as forexample, a steel strip, runs.

The substrate S may be made to run by any suitable means, depending onthe nature and the shape of said substrate. A rotary support roller 20on which a steel strip can bear may in particular be used.

Placed opposite the face of the substrate S which has to be coated thereis a small coater or extraction chamber 7 provided with a narrow slot,the length of which is close to the width of the substrate to be coated.This chamber may for example be made of graphite and may be mounted,directly or otherwise, on an evaporation crucible 3 that contains theliquid metal to be deposited on the substrate S. The evaporationcrucible 3 is continuously recharged with liquid metal via a pipe 4connected to a melting or recharging furnace 5 which is placed beneaththe extraction chamber 7 and is at atmospheric pressure. An overflowpipe 6 also connects the evaporation crucible 3 directly to therecharging furnace 5. The elements 3, 4, 5 and 6 are heated totemperatures high enough for the metal vapor not to condense or themetal not to solidify on their respective walls.

The evaporation crucible 3 and the liquid metal recharging furnace 5 areadvantageously provided with an induction heater 30, 50, respectivelywhich has the advantage of making the stirring and the compositionhomogenization of the metal alloy bath easier.

When it is desired to operate the facility 1, the composition of themetal alloy that it is desired for deposition on the substrate is firstdetermined. Then the composition of the bath for obtaining, inequilibrium with this bath, a vapor having the composition of theintended coating is determined. Ingots L of a metal alloy having thisprecise composition are produced, and are then introduced continuouslyinto the recharging furnace 5.

Once the ingots L have melted, the evaporation crucible 3 and the pipe 6are heated and then a vacuum is created in the evaporation crucible 3.The liquid metal contained in the recharging furnace 5 then fills theevaporation crucible 3. During the operation of the device, a constantlevel of liquid metal is maintained in the evaporation crucible 3 byadjusting the height between the evaporation crucible 3 and therecharging furnace 5, or by activating a liquid metal pump P. Acirculating pump installed on the overflow 6 makes it possible topermanently replenish the liquid metal in the evaporation crucible 3 soas to minimize the accumulation of impurities which, after a certaintime, would greatly reduce the rate of evaporation of the metal.

The bath is thus continuously replenished and therefore always has therequired composition at any point, while still minimizing the amount ofmaterial needed to coat the substrate.

The evaporation crucible 3 is itself provided with heating meansenabling the vapor to form and to feed a JVD coater including theextraction chamber 7, which sprays a sonic vapor jet onto the runningsubstrate S.

Surprisingly, it has been found that spraying a sonic metal vapor jetonto a substrate makes it possible to obtain a coating of an AB alloywith nanoscale mixing of the elements A and B. This result is extremelyimportant in terms of corrosion resistance as, in this case, nomicro-cell can form on the surface of the AB alloy coating when this isin contact with liquid condensates.

The sonic jet outlet orifice may have any suitable shape, such as a slothaving dimensions that can be adjusted lengthwise and widthwise forexample to accommodate the desired range of evaporation. This processthus makes it possible for the width of the vapor outlet orifice to beeasily adapted so as to maintain a sonic jet within a wide range ofevaporated metal surface temperatures and therefore a wide range ofevaporation rates. Furthermore, the possibility of adapting its lengthto the width of the substrate to be coated makes it possible to minimizethe loss of evaporated metal.

In a second embodiment as shown in FIG. 3, a facility 11 comprises avacuum deposition chamber 12 similar to the chamber 2. An evaporationcrucible 13 is placed under the vacuum chamber 12 and is connected via apipe 14 thereto.

A recharging furnace 15 is placed alongside the evaporation crucible 13,the two components sharing a common wall 16 pierced by a communicationopening 19 placed below the level of the metal alloy bath but above thebottom of these components so as to prevent any impurities that settleat the bottom of the recharging furnace 15 from being introduced intothe evaporation crucible 13.

The evaporation crucible 13 is moreover placed in a confined chamber 18,placed outside the vacuum chamber 12.

The pipe 14 feeds a JVD coater 17, similar to the coater 7.

In the same way as described above with respect to FIG. 2, thecomposition of the coating which it is desired to obtain on thesubstrate is first determined and then deduced from this is thecomposition of the metal bath that has to be present in the evaporationcrucible 13, and therefore the composition of the metal ingots L withwhich the recharging furnace 15 has to be fed.

The ingots are placed in the recharging furnace 15, which is providedwith an induction heating system. As they melt, the metal alloy passesfrom the recharging furnace 15 to the evaporation crucible 13 via theopening 19. The evaporation crucible 13 is itself provided with aninduction heating system that enables a metal alloy vapor having therequired composition to be generated. This vapor is then conveyed to theJVD coater 17 via the pipe 14, which is advantageously provided with avalve V for regulating the vapor flow rate.

By having a communication opening 19 between the recharging furnace 15and the evaporation crucible 13 it is possible to feed the evaporationcrucible 13 and provide a constant circulation between these twocomponents, thereby ensuring that a constant composition is maintainedat all points in the bath contained by the evaporation crucible 13.

The process according to the invention applies more particularly, butnot solely, to the treatment of metal strips, whether precoated or bare.Of course, the process according to the invention may be employed forany coated or uncoated substrate, such as for example aluminum strip,glass strip or ceramic strip.

The process will more particularly be applied to substrates liable tosuffer a deterioration in their properties during a diffusion heattreatment, such as bake-hardening steel strip that contains largeamounts of carbon in solid solution, which must not precipitate beforethe steel has been formed by drawing or any other suitable process. Byimplementing the process according to the invention it thus makes itpossible to make metal alloy deposition compatible with mostmetallurgies.

A further object of the present invention includes obtainingzinc-magnesium coatings. However, the process is not limited to thesecoatings, but preferably encompasses any coating based on a metal alloythe elements of which have evaporation temperatures not differing bymore than 100° C., as controlling their respective relative content isthen facilitated.

For example, mention may thus be made of coatings made of zinc and otherelements, such as chromium, nickel, titanium, manganese and aluminum.

Moreover, although the process and the facility according to theinvention are more particularly intended for the deposition of binarymetal alloys, the process and facility can be adapted to the depositionof ternary metal alloys, such as Zn—Mg—Al, or even the deposition ofquaternary alloys, such as for example Zn—Mg—Al—Si.

In the case of zinc-magnesium deposition, the thickness of the coatingwill preferably be between 0.1 and 20 μm. This is because below 0.1 μm,there would be a risk that the corrosion protection of the substratewould be insufficient. The coating thickness does not exceed 20 μm as itis unnecessary to go beyond this thickness in order to have a level ofcorrosion resistance which is required, in particular, in the automotiveor construction field. In general, the thickness may be limited to 5 μm,for example, for automotive applications.

By carrying out industrial trials it has been shown that deposition bythis process advantageously achieves a high deposition rate of 5 μm ZnMgalloy coating that can be deposited on a line running at 10 m/min, witha material yield greater than 98% thanks to the targeted orientation ofthe jet.

Furthermore, the density of the coating layers obtained may beadvantageous, due to a higher vapor energy. FIG. 4 thus shows themicrostructure of a 5 μm ZnMg alloy coating deposited on a cold-rolledlow-carbon steel.

What is claimed is: 1-19. (canceled)
 20. A vacuum deposition facilityfor continuously depositing coatings formed from metal alloys comprisingat least two metallic elements on a running substrate, comprising: avacuum deposition chamber; means for running the substrate through thechamber; a sonic vapor jet coater; means for feeding the coater with avapor, the vapor having the at least two metallic elements in apredetermined and constant ratio; means for evaporating a metal alloybath to the vapor, the metal alloy bath having the at least two metallicelements, the vapor being fed the coater; and means for adjusting acomposition of the metal alloy bath, so the composition of metal alloybath is capable of remaining constant over a course of time.
 21. Thefacility as recited in claim 20, wherein the means for adjusting thecomposition of the metal alloy bath include means for feeding theevaporation means with a molten metal alloy of a controlled composition.22. The facility as recited in claim 21, wherein the evaporation meansinclude an evaporation crucible provided with a heating means and themeans for feeding the evaporation means with a molten metal alloy ofcontrolled composition include a recharging furnace connected to a metalingot feed means and is provided with a heating system, the rechargingfurnace being connected to a respective evaporation crucible.
 23. Thefacility as recited in claim 22, further including a recirculation pipefor continuously circulating the bath, the recirculation pipe connectingthe evaporation crucible to the recharging furnace.
 24. The facility asrecited in claim 23, wherein the evaporation crucible is placed in thevacuum chamber and the recharging furnace is placed outside the vacuumchamber.
 25. The facility as recited in claim 22, wherein the rechargingfurnace and the evaporation crucible are placed side by side and have acommon wall pierced by at least one opening located beneath a level ofthe metal alloy bath and above a bottom of the furnace and of thecrucible.
 26. The facility as recited in claim 25, wherein theevaporation crucible is placed in a confined chamber and the rechargingfurnace is placed outside the confined chamber.
 27. An ingot comprising:a zinc base and comprising 30 to 55% magnesium by weight; the ingotbeing an ingot supplied to the vacuum deposition facility as recited inclaim
 22. 28. The ingot as recited in claim 27, comprising 30 to 50%magnesium by weight.
 29. A vacuum deposition facility for continuouslydepositing a coating on a running substrate, the coating including ametal alloy having at least two metallic elements, the vacuum depositionfacility comprising: a vacuum deposition chamber; a substrate runningthrough the deposition chamber; a metal alloy bath including the atleast two metallic elements, a composition of the metal bath alloycapable of remaining constant over a course of time; an evaporator forevaporating the metal alloy bath to a vapor, the vapor including apredetermined and constant ratio of the at least two metallic elements;and a sonic vapor jet coater being fed with the vapor.
 30. The vacuumdeposition facility as recited in claim 29, wherein the evaporator isfed with a molten metal alloy having a controlled composition.
 31. Thevacuum deposition facility as recited in claim 30, wherein theevaporator includes an evaporation crucible having a heater and furthercomprising a recharging furnace connected to the evaporation crucible,the recharging furnace connected to a metal ingot feeder and having afurnace heater, the recharging furnace feeding the molten metal alloy tothe evaporation crucible.
 32. The vacuum deposition facility as recitedin claim 31, further including a recirculation pipe for continuouslycirculating the metal alloy bath, the recirculation pipe connecting theevaporation crucible to the recharging furnace.
 33. The vacuumdeposition facility as recited in claim 32, wherein the evaporationcrucible is placed in the vacuum deposition chamber and the rechargingfurnace is placed outside the vacuum deposition chamber.
 34. The vacuumdeposition facility as recited in claim 31, wherein the rechargingfurnace and the evaporation crucible are placed side by side and have acommon wall, the common wall including at least one opening locatedbeneath a level of the metal alloy bath and above a bottom of thefurnace and of the crucible.
 35. The vacuum deposition facility asrecited in claim 34, wherein the evaporation crucible is placed in aconfined chamber and the recharging furnace is placed outside of theconfined chamber.
 36. The vacuum deposition facility as recited in claim29, further comprising a rotary support roller supplying the substrateto the vacuum deposition chamber.
 37. An ingot comprising: a zinc baseand being 30 to 55% magnesium by weight; the ingots being supplied tothe vacuum deposition facility recited in claim
 29. 38. The ingot asrecited in claim 37, wherein the ingot is 30 to 50% magnesium by weight.